remote controlled car, speed calculator

[danielsmusic]

hi,
me and a freind are getting a fast radio controlled monster truck.
i need to make a transmiter that will transmite a pulse back to me.
i will use a bike speedo (the ones with the magnet on the wheel).
how do i sent the pulse from the magnet to me?

i need it to be ultra light, have a 1km range, and run from as less power as possible.

i have an idea i don't know weather it is posible though.
could i use the power from the magnet to power the whole circuit. it is only sending pulses so when it powers on it sends the pulse.

 

[Someone Electro]

For 1km you will hardly get that but you can get closer ranges whith radio modules.Its an module that you can drive an login sigal in an you get it back out at ther resiver in your case this is that pulse.

 

[RadioRon]

Perhaps you should put aside the goal of 1Km range. This is not reasonable with simple circuits and a goal of low power consumption. If you are willing to live with less range, you could try adapting a garage door opener as a transmitter. These usually have a digital chip in them that generates a bit stream which then turns on and off a simple LC oscillator. Your magnet could be sensed by a Hall effect switch which could then turn on and off the LC oscillator. This would give you a range of about 100 feet depending on what receiver you use.

If you are committed to the 1Km range, perhaps you could have the hall effect switch drive the tone generator button on a continously keyed GRS VHF radio. Obviously this is not lightweight and you would have to charge up the battery in the radio. And it won't last too long before running out of power. But this is the kind of radio sophistication and battery size you will need to reach 1Km. Of course there are many other ways to do this.

It doesn't make sense to have the magnet movement generate a DC voltage. If you think about, you are using power to move the magnet around in the first place. If you use the magnet movement to generate DC you won't be generating DC with 100% efficiency, so why not just tap into the power source that is causing the magnet to move in the first place. This is more efficient.

 

[danielsmusic]

well, now looking at it i don't need 1km. that is further than any field around here. and not power comsomption ether, it needs to be light as i will be racing, and i don't want to affect the speed that much.

 

[akg]

use a FM tx and Rx

hava a look at

**broken link removed**

and

**broken link removed** -> FM Booster ..

May be helpful
Thanks
Akg

 

[RadioRon]

The suggestion of an FM transmitter from AKG looks pretty good. That extra power will come in handy. However, I'm not sure I would bother with the FM booster. Most FM receivers are reasonably sensitive enough already and you always get more performance boost by improving the receiving and/or transmitting antenna than you will by adding amplification at the receiver. In my opinion you would maximize your range by first making your transmitting and receiving antennas as efficient as possible, then by boosting your transmit power, then finally by boosting your receiver signal with an amplifier.

The type of transmitter you decide to use, whether that is a simple two transistor thing operating somewhere near 100 MHz (FM broadcast) or a VHF oscillator or whatever depends entirely on what kind of receiver you want to use. Since receivers are more difficult to get right than transmitters you should pick your receiver first. If it is to be an FM broadcast receiver, then that sets your frequency choice to around 100MHz. This is a good choice since it is easy to lay your hands on an FM receiver, and at 100MHz you can have reasonably good antenna performance in a small size.

You can choose any FM microphone circuit, such as the one recommended by AKG. This will be nice and small, won't take much battery power, and will be very light if you just construct it on a bit of perfboard (ie. don't bother putting it into a box or anything, just build it up and then put tape or a plastic bag around it). When you receive its signal on an FM receiver though, you will only hear a lack of background noise when the tx is on, which isn't a very positive signal to use. However, some FM receivers have signal strength meters that you can tap into for an on/off signal. Beware that such circuits usually don't respond very quickly, so it likely wont' be very good for RPM pulses. Otherwise, you might want to consider modulating the FM mic with an audio tone so that you have something to work with at the other end. Then, at the receiver, you would use a tone decoder or other simple circuit to change your audio tone back into a pulse stream.